Multimedia services providing services in terms of transferring and providing various kinds of information by way of radio transmission are getting more active lately and a great number of wireless apparatuses have been developed and put into practical use. These services are diversified year after year, involving telephones, TVs, Local Area Networks (LANs). etc. End users are required to have different wireless apparatuses for different services to receive all services.
With the aim of improving the usability of end users who receive such services, attempts have already started to provide the services to end users anytime and anywhere, namely, in a ubiquitous manner, thus making the presence of media transparent to the users. A single terminal apparatus that implements a plurality of information transfer services, namely, a so-called multi-mode terminal is realized, but partially.
Because a ubiquitous information transmission service by ordinary radio transmission uses electromagnetic waves as its medium, a plurality of services are provided to end users by using several frequencies in a same service area; one frequency for one type of service. Therefore, a multi-media terminal is required to have capability of transmitting and receiving multiple frequencies.
For conventional multimedia terminals, a method in which a plurality of single-mode antennas, each provided for one frequency, are installed on a single wireless apparatus is used. In this method, it is needed to install the antennas separated each other by a distance equivalent to wavelength to make each single-mode antenna operate independently. Because the frequencies of electromagnetic waves that are used for services in terms of normal ubiquitous information transmission are limited to a range from a few hundred MHz to a few GHz due to the limitation of their free space propagation characteristic, the antennas must be separated each other by a distance of a few tens of centimeters to a few meters. Consequently, the dimension of the terminal becomes large and portability for the user is not satisfied. Because the antennas sensitive to different frequencies are arranged, separated each other by a distance, it is needed to install separate RF circuits connecting to the antennas for each frequency.
For this reason, it is difficult to apply semiconductor integration circuit technology and there arises a problem of high-cost RF circuits as well as the increased dimensions of the terminal. Even when the RF circuits are integrated into a whole by applying the integration circuit technology with great efforts, there is a need for connecting the RF circuit to the individual antennas separated by a distance with RF cables. By the way, the diameter of the RF cable applicable to a terminal with dimensions allowing for portability for the user is around one millimeter. Consequently, transmission loss of the RF cable in the current situation reaches a few dB/m. With the use of such RF cable, power consumed by the RF circuit increases. This causes a significant decrease in use duration of the terminal providing ubiquitous information services or a significant increase in the terminal weight due to increased battery volume and poses a problem of significantly degrading the usability for the user of the terminal.
Aside from the foregoing, two-frequency duplex antennas in which one end of a loop antenna or the material of the antennal is connected to a transmitter which transmits at one frequency and the other end is connected to a receiver which receives at the other frequency are disclosed (e.g., Japanese Patent Laid-Open No. S61(1986)-295905 and Japanese Patent Laid-Open No. H1(1989)-158805).
A two-frequency duplex antenna described in Japanese Patent Laid-Open No. S61(1986)-295905 is configured such that first and second resonant circuits respectively connected to either ends of the loop antenna which is a radiating conductor resonate with the loop antenna, wherein one resonator at one terminal resonates at a transmit frequency and the other resonator at the other terminal resonates at a receive frequency, and the transmitter is connected to the one terminal and the receiver is connected to the other terminal.
Another two-frequency duplex antenna described in Japanese Patent Laid-Open No. H1(1989)-158805 is configured such that a first resonant circuit resonating at a transmit frequency, connected between one end of the material of the antenna which is a radiation conductor and a transmit output terminal, assumes a high impedance to a receive frequency and disconnects the material of the antenna from the transmit output terminal, and a second resonant circuit resonating the receiving frequency, connected between the other terminal of the material of the antenna and a receive input terminal, assumes a high impedance to a transmit frequency and disconnects the material of the antenna from the receive input terminal.
Even for a wireless apparatus employing either of these two-frequency duplex antennas, it is needed to provide the transmitter and the receiver for each of input and output terminals (feeding points) located at separate positions for different frequencies. Thus, it is difficult to integrate both, which makes a bottleneck in downsizing the wireless apparatus.